In central heating boilers built for solid fuel combustion in which the fuel burns on a level grate, either of metal rods or on a continuously bricked hearth, the behaviour of ash causes problems when using granular fuel manufactured of artificially dried material rich in ash content. The ashes soften and partly fuse and also while cooling form solid clinker, preventing the entry of the combustion air into the fuel material level and gradually choke combustion. The maintenance interval, based on results from combustion experiments with the smaller type of boiler in use today, has been experimentally determined as only 3-4 hours when tests were carried out when peat pellets, and is using industrially processed fuel no gain will be achieved in comparision with the traditional wood and peat fuels.
Fuel feeding and fuel combustion equipments to be installed as additional equipment to the boiler and intended for use with granular fuels are stokers, meant for combustion of wood chips, and various kinds for crucible and crucible burners, or equipment for other granular fuels modified from these, such as equipment for peat pellets. Stokers and crucible burners are installed in the furnace so that the gas flame is directed vertically upwards so that the combustion equipment takes up part of the furnace space.
It is known that fuel feeding equipment to the grate is also in use in which additional air is led by the feeding equipment to the gas combustion space above the grate. This kind of feeding equipment is illustrated for example in SE Patent Publication No. 109 115. The feeding equipment according to the patent publication cannot be used as an independent burner, as the combustion of the fuel occurs on a separate grate.
In general the ashes are meant to be extracted from the stoker fuel combustion space by the screw conveyor and under the influence of the combustion air. Whilst fusing and further cooling down the ashes form solid particles which adhere to the walls of the combustion chamber. While the fuel feed continues and the ashes lessen the stoker combustion space and prevent the flow of combustion air, the screw conveyor thrusts the fuel over the stoker edges to the bottom of the boiler.
The feed of combustion air into the combustion crucible occurs through the grate at right angles to the fuel layer. It is intended that ash is removed together with the gas flow as dust. The smallest particles of the fuel are taken with the gas flow before they reach the grate and float down to the bottom of the boiler as they impinge on the boiler walls. At the bottom of the boiler the temperature is so low, that the fuel that has scattered there does not burn and thus causes significant losses. In the combustion tests on peat pellets it was verified that the peat loss caused by the fuel fallen into the ashes in a typical crucible burner was at least 10% as compared with expected thermal value. When burning peat pellets the ashes, as they fall down onto the crucible grate and cool down further, form solid clinker, which gradually clogs the whole burner and thus prevent combustion.